Worth considering both P. malacea – densely tomentose on upper surface, which I don’t see here – and more promising, P. elisabethae – hairless on upper surface, thick (esp. in exposed situation like this), stubby thick rhizines that tend to be arranged in concentric rings near the margins, with abundant regeneration lobules in most specimens. If you find an apothecium, P. elisabethae should have flat “horizontal” apothecia like P. horizontalis, not the usual “hands up!” or saddle- or finger-shaped apothecia of P. rufescens.